It has nothing to do with "keeping up appearances". The slackers are easily and quickly identified (usually just from the indicator that they chose to WFH 100% of the time). The problem is that, as I mentioned, it is ridiculously hard to remotely correct a slacker situation even after it has been identified.
so everyone knows the problem people, but it's hard to sanction/fire them? again sounds like managment/organizational failure. Surely judging performance of WFH employees and making efforts to help them is a surmountable management problem
Have you ever worked in a management position at a decent sized company? Firing someone is not an easy task. Unless it's an egregious case (violation of laws/policy, sexual assault, etc), firing someone, even someone that literally produces zero work product, can take months or even a year. That's a slacker that is going to sit on your payroll, disrupting your teams flow, and eating your budget for months/a year while you build your case for firing.
Then, even after you fire them (or let's even say by some miracle you were able to fire them quickly), it costs a lot of money and time to hire someone new. That's yet more time and budget that isn't going into your team, and your team is also a person short while you go through the process of hiring and then training.
It is leagues and leagues better for "the slacker situation" to not be a problem at all, and it is much less likely to be a problem if prospective slackers aren't given the opportunity to slack off at home to begin with.